Ice cubes

Question:

You have an old-fashioned refrigerator with a small freezer compartment that could hold seven ice cube trays stacked vertically, but there are no shelves to separate the trays. You have an unlimited supply of trays, each of which can make a dozen cubes, but if you stand one tray on top of another before it’s frozen, it will nest part way into the one below and you won’t get full cubes from the bottom tray. Given these constraints, what is the fastest way to make ice cubes?

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Solution:  Many situations require using the outcome of the process under consideration as a catalyst to solve the problem. The solution to this puzzle uses one of the outcomes. By using frozen cubes as spacers to hold the trays apart, you can make 84 cubes in the time it takes to freeze two trays. Here’s how: Fill one tray, freeze it, and remove the cubes. Place two cubes in the opposite corners of six trays, and fill the rest with water. Freeze all six, plus a seventh you put on top, at the same time.

Solution: Use 12 frozen ice cubes as spacers to hold the trays apart.

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